.topbar {
    height: 42px;
    width: 100%;
    background-color: #000;
    color: white;
    line-height: 42px;
    font-size: 15px;
    text-align: center;
}
.bg {
    /* 脱离文档流 */
    position: absolute;
    z-index: -1;
    width: 100%;
    height: 1200px;
    background: linear-gradient(189deg,rgb(57,15,199) 13%,rgb(40,35,77) 98%);
}